V22A-01 INVITED
From Map Unit to Magma Chamber: Understanding the 2006 Eruption of Augustine Volcano
In 2006, Augustine Volcano once again sprang to life and erupted ~70 x 106 m3 of magma during three eruptive phases. Variations in magma composition, eruptive style, and deformation of the edifice provide clues to the ascent and interaction of magmas prior to and during the three-month-long eruption. Genetically unrelated end members basaltic andesite (56.5 wt% SiO2) and dacite (63.3 wt% SiO2) bracket erupted magma compositions. Products from all three eruptive phases contain both end members though proportions varied with time, and many pyroclasts have mixing textures. Initial Vulcanian explosions in mid-January 2006 (explosive phase) erupted small-volumes (~14 x 106 m3 DRE) of basaltic andesite as ash fall and pyroclastic flows. In late January, explosions transitioned to continuous "boil over" at the vent (continuous phase), producing relatively voluminous (28.5 x 106 m3 DRE) block-and-ash flows that are rich in dacite and banded clasts. In early February, explosive activity gave way to effusion of basaltic andesite lava (effusive phase; 32 x 106 m3 DRE) highlighted by a pulse of increased effusion from March 7–14. Effusive activity ceased by the end of March. Geophysical and petrologic evidence lead us to a hypothetical series of magmatic processes that drove the eruption. The presence of amphibole in all eruptive products indicates that magma storage must have been within the amphibole stability field at depths greater than ~4 km, in agreement with elevated water (2 - 4 wt% by difference) and chlorine (2400 - 4900 ppm) contents in melt inclusions. Precursory unrest took the form of six months of volcano-tectonic earthquakes and island-wide uplift and radial displacement (recorded by GPS), all centered near sea level. Preliminary textural analysis of the explosively erupted basaltic andesite indicates that it did not undergo the degree of decompression-driven crystallization expected if it had accumulated at sea level (~40 MPa) for longer than a few days. This suggests that the precursory geophysical signals were not caused by magma accumulation, but perhaps by gas released from and preceding the ascending magma. We hypothesize that gas-rich basaltic andesite rose from the mid-to-lower crust, through a mid-to-upper crustal dacite magma body, and toward the surface. Mixing textures suggest that the basaltic andesite that erupted during the explosive phase also entrained dacite magma and created a pathway for the mid-to-upper crustal dacite to ascend. A ~10- km-deep deflationary source coincided with the continuous eruption of dacite in late January and early February. Unrimmed amphiboles in continuous-phase dacite are consistent with rapid ascent from within the amphibole stability field. Following continuous dacite eruption, the mostly degassed "tail" of the basaltic andesite continued its ascent. Some basaltic andesite erupted effusively, but shallow inflation suggests that some accumulated in the shallow crust. Subsequent shallow deflation suggests that this basaltic andesite remobilized during the mid- March lava extrusion.
V22A-02 INVITED
Pumice textures, magma mingling and fragmentation processes at Stromboli volcano (Italy)
A paroxysmal eruption of Stromboli volcano on April 5, 2003, lasted about ten minutes, and comprised 4 different phases. The eruption climax was reached during phase 2, where the average magma eruption rate was 2.6*106 kg/s (Rosi et al. 2006). The erupted juvenile material included both low porphyritic (LP) and highly porphyritic (HP) magmas (Metrich et al., 2005; Corsaro et al., 2004). LP magma was predominant during the climactic phase, strongly decreased in the successive explosions of Phase 3, and was negligible in the final ash emitted during Phase 4. Textural and physical features of the juvenile clasts emitted during the climactic phase were analyzed to obtain information about eruption dynamics and fragmentation processes. We studied a population of pumices (diameter size 2-3 cm) deposited about 1.5 km from the vent, on the Punta Lena coast. The clasts are equant to slightly elongated with vesicularity ranging from 40 to 85%, and phenocryst content ranging from <5 to 50%. They have glassy groundmass, with sparse microlites. Vesicularity and mean vesicles size increase from the rim to the inner portion of the clasts, the result of postfragmentation expansion. Some exceptionally expanded clasts display a central main cavity surrounded by a microvesicular glassy rim less than 1 cm thick. The quenched rims contain a high number density of small uniform vesicles suggesting very fast rates of magma ascent and bubble nucleation. Most clasts show intimate mingling between the two, compositionally distinct, magma types. In LP glass vesicles are mostly spherical and display homogenous features, sub mm diameters, and show little coalescence. In HP magma the vesicle population is heterogeneous: groundmass vesicles have sub mm size, spherical shapes and little coalescence; larger, mm sized vesicles generally have elongated or convoluted shapes often adapting to the surrounding phenocrysts.As a conclusion, our data indicate that the dynamics of the April 5 2003 Stromboli paroxysm was likely originated by magmatic fragmentation of of volatile-rich LP magma subjected to fast ascent and decompression eventually rising through, and effectively mingling with a superficial reservoir constituted by degassed, more crystalline, texturally mature HP magma.
V22A-03 INVITED
Texture and Composition of Pumice and Scoria Provide New Insights into the Dynamics of Explosive Eruptions at Campi Flegrei (Italy)
Campi Flegrei (CF), a nested caldera located west of the densely-inhabited city of Naples, has been the site of volcanic activity for the past 60 ka BP. The last eruption occurred in AD 1538, but geochemical and geophysical signals have been monitored for the last tens of years clearly indicating that the magmatic system is still active. Volcanic risk mitigation in this area is therefore a primary goal of the scientific community. Here we combine new and literature data on the texture and composition of pumice and scoria clasts of selected CF eruptions with the goal to investigate their relationship to, and implications for, the eruption dynamics. We focus on three events with decreasing eruption intensity and magnitude: the Campanian Ignimbrite (39 ka), Agnano Monte Spina (4.1 ka) and Monte Nuovo (AD 1538) eruptions. Previous studies indicate that phenocryst and major element bulk compositions, as well as the original volatile content, are comparable in samples from the investigated eruptions. From these observations we derive that similar pre-eruptive physico-chemical conditions governed magma storage in the shallow crust before the occurrence of the above eruptions. Our investigation displays however a significant heterogeneity among the studied samples in terms of crystal and vesicle texture and abundance, composition of microlites and glassy groundmass, residual groundmass water content, and, finally, rheological properties of the related magmas. We ascribe such textural, compositional and rheological variability to different mechanisms of volatile exsolution, separation from the melt and outgassing accompanying magma ascent along the conduit and generated primarily by changes in the magma ascent rate. Low ascent rates allow open-system degassing, groundmass crystallization in response to water exsolution and the development of permeable flow pathways through which volatiles escape non-explosively to the surface potentially decreasing the overall eruption explosivity, as in the case of Monte Nuovo. Faster ascent rates instead inhibit crystal nucleation and growth and promote syn-eruptive vesiculation under closed-system conditions, which, in turn, translates into eruptions of higher explosivity (i.e. Campanian Ignimbrite or Agnano Monte Spina). Because our study indicates that magma ascent rate was the key-parameter in driving the diversity of eruptive intensity and style of activity observed at CF, we conclude that future research should be aimed at quantifying factors generating changes in such parameter and at integrating geochemical and geophysical models for a comprehensive understanding of the eruption dynamics in this hazardous volcanic area.
V22A-04
Leucite Crystallization in 79AD Vesuvius Magmas Inferred from Decompression Experiments
Small leucites are ubiquitous in the phonolite and tephriphonolite magmas erupted in 79 AD at Vesuvius. They show a narrow size range (27±5 μm) throughout most of the eruptive sequence and their habit is generally euhedral. While plagioclase in calc-alkaline magmas may crystallize during ascent (e.g. Mt St Helens), the mechanism of leucite formation in peralkaline magmas is unclear. Decompression experiments on 79 AD samples may yield valuable information on their origin. In this study, single step (SSD) and multiple step decompression (MSD) experiments were conducted using rapid-quench-capable cold seal vessels with natural EU1 and EU2 phonolitic pumices as starting material. Prior to decompression, samples were equilibrated under water-saturated conditions at 150 MPa and 800, 825, and 850°C for 5-7 days, which is above leucite stability curves for both EU1 and EU2. The SSD runs involved rapidly decompressing the experiments followed by holding at 25 MPa, within the leucite stability field for all temperatures, for 5 min-7 days. The MSD runs involved decompressing the samples in a step-wise manner, where each 5 MPa pressure drop was followed by hold time of 20 seconds, replicating an ascent rate of 10 m/s. All SSD runs contain leucites with skeletal habits, regardless of the length of the hold time at 25 MPa, which suggests diffusion-limited growth and crystallization in a nucleation dominant regime after a very large effective undercooling. In contrast, the MSD runs using EU1 starting material at 800 and 825°C produced euhedral leucites similar to those in natural samples. At 850°C, the MSD runs failed to crystallize leucite in either EU1 or EU2. This suggests that leucites in the 79AD magmas could have formed either in the magma chamber, or during ascent. However, if they formed in the chamber, P and T conditions could not have exceeded 100 to 150 MPa and 800 to 850°C, assuming water saturation. Alternatively, if they formed during ascent, magma temperature could not have exceeded 825°C, assuming a mostly linear decompression path. More work comparing leucite size distributions between the experiments and natural samples will provide constraints on their formation in the 79 AD phonolite.
V22A-05
Constraining Pre-eruptive Pressure/Temperature Variations, Transition From Chamber to Conduit, and Crystal Growth Rates: a SIMS Examination of Plagioclase Phenocrysts
Explosive volcanic eruptions may be triggered by physical changes in the volcanic conduit that include pressure (P) and temperature (T) variations, volatile loss, and crystal growth. These changes cannot be directly witnessed, and so must be inferred through the examination of eruptive products. Experiments show: 1) plagioclase An content varies as a function of P and T and 2) lithium is a rapidly diffusing trace element within this phase. Clasts were examined from two explosions, ExpA and ExpB, which occurred in a series of 88 vulcanian events at Soufrière Hills volcano, Montserrat in the fall of 1997. Here, we show that SIMS depth-profiling of plagioclase crystals from the clasts reveals An and Li variations that indicate late-stage, decompression-induced growth, and reveal variable P and T in the conduit. These data are consistent within clasts from the same vulcanian event but differ between the two explosions. An contents increase by 23 and 13 mol% within the final 25 μm of crystal growth for plagioclase derived from ExpA and ExpB, respectively. Overpressures that would generate the measured An increases exceed geophysical estimates for the 1997 vulcanian explosions by several tens of MPa, suggesting there was a combined effect of increasing P and T in the conduit (i.e., conditions were not isothermal). When the contribution of latent heat is included, a 100°C increase for ExpA and a 50°- 60°°C increase for ExpB, calculated overpressures are 1-33 MPa, covering the estimated range. Inflection of the near-edge Li signal allows us to identify when the crystals moved from the chamber to the conduit, where decompression-induced exsolution of volatiles began. Crystal growth rates within the conduit were calculated from the depth of Li inflection and the average time between eruptions, and they are consistent, within an order of magnitude, with values obtained from independent decompression experiments. Chemical variations in the rims of plagioclase phenocrysts preserve a geochemical record of the physical conditions in the conduit between vulcanian explosions at Soufrière Hills, allowing us to directly quantify the amount of crystal growth and the magnitude of P/T variations preceding an eruption.
V22A-06
2006-2007 eruptions of Bezymianny volcano, Kamchatka: Petrological snapshots of the compositionally changing magma system
Bezymianny volcano, Kamchatka has been in a state of almost continuous eruptive activity since October 1955. On March 30, 1956 a sudden collapse of the edifice triggered a devastating directed blast followed by a vigorous Plinian eruption, which destroyed the summit of the volcano and formed a 1.3-km-wide crater. Within weeks the volcano started rebuilding its edifice through extrusion of the dome in the middle of the crater, intermittent collapses of the dome, and associated block-and-ash flows. By mid 70s, as the volume of the dome increased, the dome-building extrusive activity became complemented by short explosive events with pyroclastic flows and surges followed by effusions of lava flows. By late 90s the explosive eruptions became remarkably regular with 2 events per year. The change in the eruptive behavior correlates with a gradual change of the whole-rock composition of erupted magmas, which become progressively more mafic, i.e. 60.9 wt.% SiO2 in 1956 and 56.7 wt.% SiO2 in May 2006. Hornblende phenocrysts, abundant in the March 30, 1956 magma, almost disappeared in the most recent erupted products. The 1955-2007 sequence of erupted products of Bezymianny provides an excellent opportunity to investigate the response of the magma system to the collapse of the edifice. The most recent 2006-2007 products serve as snapshots of the compositionally changing magma system and may provide an opportunity to determine mineral growth rates and ages of individual phenocrysts through direct comparison of new and old erupted products. Our ongoing petrologic study and phase equilibria experiments focuses on the pre-eruptive magma conditions for individual eruptive episodes, as well as their variations since the onset of the eruptive activity at Bezymianny in 1955.
V22A-07 INVITED
Columnar jointing in non-welded Cerro Galan ignimbrite: Paycuqui, Argentina
Well-developed columnar joint sets are thought to occur most commonly in moderately to densely welded facies of ignimbrites. Columnar joints in welded deposits form when sintering between juvenile grains provides sufficient cohesion to allow thermal volume contraction to be accommodated in brittle fracture. However, well- developed columnar joints can also form in unwelded deposits, where cohesion is produced by high temperature devitrification of the deposit. Such is the case at Paycuqui, Argentina, in the Cerro Galan ignimbrite. The Cerro Galan ignimbrite is a very large volume rhyodacitic pyroclastic flow deposit (>1000 km3), which erupted ~2.3 Ma. At the Paycuqui locality, 29 km west of the caldera margin, the flow is valley-confined and is over 30 m thick. The deposit is massive, pumice-poor, lithic-poor, crystal-rich, and contains well-developed joint sets in the upper 15m. Remant magnetization analyses of incorporated lithics indicate high temperature emplacement. However, welding deformation and sintering textures are absent. Instead, textural differences between the nonjointed base and the jointed top of the ignimbrite are coincident with devitrification variations, inluding dominant cryptocrystalline sanidine and crystobalite that increase in abundance upwards in the section. The presence of a 1mm wide, more devitrified zone along columnar joint faces constrains the timing of high temperature devitrification. Devitrification must have begun prior to brittle fracture and continued after joints had formed. The morphology of joints at Paycuqui differs from well-described joint sets elsewhere. The average diameter of polygons is 0.75 m. Intersecting columnar joint surfaces at Paycuqui form polygons with fewer sides (mean of 4.5 sides) than on many lava flows or than predicted in models. These polygonal patterns vary within the flow due to the presence of vertical, rose-shaped, and radiating joints. Such complexities may result from fumarolic vapor rise.
V22A-08
The Influence of Grain Size and Crystal Content on Rheology and Deformation of Pyroclastic Material
Pyroclastic deposits undergo variable degrees of sintering, viscous deformation of particles and loss of pore space, which combine to produce the dramatic textural variations that define welded facies. We here investigate the effects of grain size and crystal content on the rheology and welding of pyroclastic material.Uniaxial deformation experiments were conducted using sintered cores of natural rhyolite ash under conditions consistent with welding. Experiments were done in the University of British Columbia Volcanology Deformation Rig (VDR). This apparatus is designed to run experiments relevant to volcanology, by supporting low-load, high temperature, deformation experiments (Quane et al., 2004). We ran experiments at constant displacement rate (2.5.10-6 m.s-1), under ambient water pressure ("Dry"), at temperatures of 850 and 900°C and to maximal strain of 50%. Grain-size effect was investigated using sintered cores from three different sieving fractions of Rattlesnake Tuff (RST, Eastern Oregon, USA) ash: fine ash (grain size < 0.6 mm), coarse ash (0.6 to 2mm) and row unsieved ash. The effect of crystal content was explored using cores of sintered unsieved RST ash, variably enriched in crystals of feldspars and quartz.Unsieved and fine ash cores suffered higher total porosity reduction than coarse ash cores during deformation experiments. For cores of unsieved ash, porosity loss is facilitated by mechanical compaction, which includes orientation and organisation of different size clasts to a compact assemblage, without any deformation of individual particles. Isolated porosity decreases faster than connected porosity in coarse and fine ash cores, whereas cores of raw ash mainly loose connected porosity. This is also consistent with mechanical compaction for cores of unsieved ash, in which isolated porosity of weakly deformed individual pumice clasts or glass shards is maintained. Increasing strain causes a reduction in porosity and correlates with increase in effective viscosity for all grain sizes. Cores of unsieved raw and fine ash, for which porosity reduction is highest, show the highest values and greatest increase in effective viscosity (1011.2 – 1012.4 Pa.s at 850°C for raw ash and 1011.1-1011.8 Pa.s for coarse ash).Crystals reduce shard-shard contacts and thus sintering of glass shards. They act as a brake to welding. We found that >15% crystal content inhibited sintering in a sample that welded under the same experimental conditions when phenocryst depleted (phenocryst content around 1% in whole Rattlesnake Tuff ash). Reference: Quane, S.L., Russell, J.K., and Kennedy, L.A. (2004). A low-load, high-temperature deformation apparatus for volcanological studies. American mineralogist, 89, 873-877.